Nanostructured materials, such as carbon nanotubes (NTs) have been the subject of much research in recent years because their unique electronic, mechanical and physical properties can be used in all areas of science.1,2 Nanostructured tubes are becoming increasingly important because their mechanical properties, notably strength, stiffness, and toughness, make them useful in a variety of applications. Composites made from carbon nanotubes have superior strength and stiffness per weight, and thus are already being used in aerospace and sporting goods applications. Carbon nanotubes are also excellent electron emitters, emitting electrons at very low voltage, which makes them particularly useful in the field of flat panel displays. Their good electron conductivity and high thermal conductivity also make carbon nanotubes ideal materials for making anti-static and/or anti-corrosion coatings as well as thermal conductors. Additionally, carbon nanotubes can be used in electronic circuits where silicon and other standard semiconductor materials do not work.
Carbon nanotubes are particularly stable because of the strength with which the carbon atoms bond together. In nanotubes, the carbon atoms arrange themselves in hexagonal rings, similar to the arrangement in graphite. In fact, a nanotube resembles a sheet or several stacked sheets of graphite rolled into a seamless cylinder. Unfortunately, carbon nanotubes are difficult to dissolve or disperse in most organic or inorganic solvents because of their long structured features, large molecular size, or severe aggregation. The common agents used to help disperse carbon nanotubes are surfactants, which, however, can only increase the dispersibility to a limited extent, and surfactants do not affect the solubility of carbon nanotubes. Attaching alkyl groups to the surface of carbon nanotubes can increase their solubility to some extent. This approach, however, requires tedious chemical reactions, and the chemical functionalization may alter the structure or affect properties of the carbon nanotubes. In another approach, carbon nanotubes were wrapped with a helical polymer, through which they were brought into other solvents. This method, however, can only be applied in very limited circumstances. The size of soluble nanotubes depends upon the size of the helices of the polymer, and it is tedious to synthesize these polymers. This difficulty in dispersing or dissolving carbon nanotubes limits the ability to incorporate carbon nanotubes in other organic or inorganic materials as well as the ability to manipulate them chemically and characterize them quantitatively.
Some solution properties of carbon nanotubes have also been studied, aimed at their chemical modification and functionalization.3-6 Several methods have been reported for making solubilized NTs, including attachment of long alkyl chains3 and incorporation with polymers.4,5 Haddon et al., in U.S. Pat. No. 6,187,823, disclose a method of dissolving single-walled nanotube carbons in organic solvents by directly functionalizing the nanotubes with amines or alkylaryl amines having an uninterrupted carbon chain of at least five and, more preferably, nine carbon atoms.
Despite those attempts, the dissolution of pristine carbon nanotubes, to our knowledge, has not been realized.